Microwave transmitter-receiver

ABSTRACT

Device for emission-reception of a stream of electromagnetic waves, in which an emission unit ( 1 ) and a reception unit ( 2 ) are each supplied with light energy passing through an optical guide ( 13 ). This light may be stopped by an optical gate ( 19 ). Otherwise, it reaches a micro-laser ( 4 ) that illuminates a photo-conducting sheet ( 5 ) on which an antenna ( 3 ) is located. The portions ( 6, 7 ) of the emitter antenna ( 3 ) at different potentials are then short circuited and a stream of electromagnetic waves is emitted. The wave stream passes through a medium to be studied and is returned to the receiving unit ( 2 ) in which it is sampled, recorded and analyzed to determine the composition of the medium through which it had passed. The wave stream has a wide frequency band and the opto-electronic device can be used to make a very compact system.

DESCRIPTION

This invention relates to a microwave emitter-receiver device.

It may be used for miscellaneous applications and particularly tomonitor physical or chemical changes in a surrounding environment or todetect when an object passes. It is known that radiation passing throughthis environment is subject to disturbances that depend on itscomposition; the emitter then sends radiation in the form of a definedsignal and the measurement consists of analyzing the signal received bythe receiver after being modified by the medium through which it passes.These examination processes may be used continuously for applications inwhich risk areas are monitored and when carrying out tests; disturbancesto environments through which radiation passes may be solid objects, oralso and very frequently gasses, atmospheric pollutants, leakage gases,fire and firing plums. These devices may be used with measurementbenches or with alarm beacons.

Infrared radiation, visible light and ultraviolet radiation have alreadybeen used with this type of device, but they have the disadvantage thatthey become inoperative under bad conditions; surrounding thermalradiation sources also produce infrared radiation that disturbs themeasurements, and rain absorbs light radiation. These types ofdisturbances prevent detectors that analyze the received signal fromseeing the signal.

Therefore it is preferred to use other wave types. Some systems includeemission of a carrier wave within a restricted frequency range, whichhas the disadvantage that it only enables reduced observations. It isusually preferable to supply a wave stream extended over a widefrequency range, which imposes the use of a brief pulse so that a largeproportion of the signal emitted is located in high frequencies. Arecent illustration of a radar system with ultra-short pulses and a widefrequency range is given in the article by W. M. Boerner and J. S. Verdi“Polarimetric UWB Radar/Sensing & Imaging” at the UWB-SP-2 Conference inAlbuquerque in May 1996. Systems with electronic, opto-electronic ormixed triggering of the wave stream have been proposed.

The invention relates to an emitter-receiver device that can be used inhigh frequency spectroscopy (of the order of a gigahertz or a terahertz)and which has a particularly easy and compact design. It comprises anemitter and a receiver of electromagnetic waves with an antenna. Thewave stream is triggered opto-electronically, using a light pulsegenerator system that illuminates a photo-conducting layer on which theemitter antenna is deposited or printed; a short circuit is created inthe antenna which becomes active and emits the wave stream. Light pulsesare also transmitted to the receiver, the receiver antenna being laidout in the same way on a photo-conducting layer, and also activates thisreceiving antenna so that it can collect the wave stream after it haspassed through the environment being studied and then transmit it tomeans of recording or using the stream. The receiving antenna usuallyneeds to be activated slightly after the emitting antenna to synchronizethe process by absorbing the time necessary for the wave stream to passbetween the antennas. A prior opto-electronic device that can be usedfor the same applications is described in U.S. Pat. No. 4,855,749 A, butit is not the same as the invention and its performances are much lower,as will be seen in more detail later.

All elements of the device may be placed on a single support board or ona small number of such boards, and a single electricity power supply ispossible both for the emitting antenna and for control, usage andrecording means, and for the light pulse generation means which could bea laser. In this case, its operation may be made discontinuous by anopto-electronic gate that chops its light radiation or by an elementthat varies the impedance of optical lines through which its lightpasses.

In its most general form, the invention relates to a waveemitter-receiver device to examine a medium through which the wavespass, comprising an emitter with an antenna and a receiver with anantenna, a wave generation device connected to the emitter, and a signalreception and usage device connected to the receiver; the antennas areformed of distinct portions and are laid out on a photo-conductinglayer, the wave generation device comprises an electricity power supplyinitiating a potential difference between the portions of the emitterantenna, and the wave generation devices and signal reception and usagedevices comprise means of supplying light pulses to the photo-conductinglayers between the portions of the antennas; all this is similar to thedevice in U.S. Pat. No. 4,855,749, but the invention also involves theuse of micro-lasers fixed on photo-conducting layers in order to formintegrated receiver and emitter elements, whereas in the previous patentexcitation is obtained by ordinary lasers which are large and difficultto adjust correctly. Note that with this type of opto-electronic deviceit is difficult to suitably transmit a wave stream due to its intensityand its frequency band. However the invention offers a device composedof emitter and receiver elements for which the adjustment will notchange, due to the use of micro-lasers that are perfectly compatiblewith the stacks of photo-conducting layers. Another advantage is thatthese emitter and receiver elements may be moved without difficulty ifthe tests are to be carried out in different locations.

The invention will now be described in more detail using the followingfigures attached for illustrative and non-restrictive purposes, in orderto better understand and describe other characteristics and qualities:

FIG. 1 is an overview of an embodiment of the invention;

FIG. 2 illustrates some details of another embodiment of the invention;

FIG. 3 is a view of another embodiment of the invention;

FIGS. 4 and 5 illustrate emitted and received wave streams;

FIGS. 6 and 7 illustrate use of these wave streams for measurement;

and FIG. 8 illustrates a variant of FIG. 1.

The variant embodiment in FIG. 1 was designed to measure signalsreflected by the medium being studied. Therefore it comprises an emitter1 and a receiver 2 placed side by side and in parallel, each of themhaving an antenna 3 pointing towards the front of the instrument, amicro-laser 4 located behind the antenna 3, and the assembly is placedon a sheet comprising a photo-conducting layer 5 as the upper layerwhich may be made of silicon doped with oxygen, or better cadmiumtelluride or gallium arsenide.

The antennas 3 are composed of at least two separate conducting parts 6and 7 (made of aluminum or a chromium and gold alloy) deposited on thephoto-conducting layer 5, the shape of which depends on theelectromagnetic waves that they will be required to guide. In the caseshown, they are in the shape of a parabola and the distance between themincreases as the distance from micro-laser 4 increases. Spiral or othershaped antennas could be designed, as is well known. Parts 6 and 7 ofthe emitter antenna 3 are connected to two lines 9 and 10 respectivelyof a DC power supply 11, so that a potential difference can be imposedon them at all times. If the photo-conducting layer 5 is illuminated, itconducts electricity and a short circuit arises between the two parts 6and 7 of antenna 3 which triggers emission of an electromagnetic wavestream.

The light radiation used for this purpose must excite the electrons inthe atoms of the photo-conducting layer 5 so that they pass from thevalence bands to the conducting band. This plasma can only be obtainedif the radiation photons have sufficient energy to enable thisexcitation depending on the chemical nature of the photo-conductinglayer 5.

The purpose of the micro-laser 4 is to provide the required illuminationunder good conditions. It is controlled by a laser diode 12 through anoptical guide 13 including a fork, one branch 14 of the fork comprisinga single line leading to the micro-laser 4 of the emitter 1; the otherbranch 15 of the optical guide 13 leads to the micro-laser 4 of receiver2 and comprises optical delay lines 16 that can also illuminate thephoto-conducting layer 5 and create a short circuit between parts 6 and7 of the antenna 3, but with a delay. Since the delays of lines 16 aredifferent, the antenna 3 of receiver 2 becomes active at differentsuccessive instants. Shielding 17 is placed between emitter 1 andreceiver 2 to avoid electromagnetic interference between the twoantennas 3.

The electricity power supply 11 supplies power to the various elementsin the instrument including the laser diode 12 and a control element 18used to adjust operation of an opto-electronic gate 19 located on theoptical guide 13. This is a component such as a Pockels cell that canblock off the optical guide 13 to light from the laser diode 12, orenable it to pass and touch the micro-lasers 4, according to an internalmodification, for example concerning the refraction or extinguishingindex which is imposed on it by an electric field or a polarizationvoltage set up in an adjacent crystal polarized by the electroniccontrol 18. A microprocessor 20 also receives energy from the powersupply 11 and is assigned more precisely to recording and analysis ofsignals received by the receiver 2, through an amplifier stage 21 andconnection lines 22 leading to the micro-laser 4 of receiver 2. Finally,the electricity power supply 11 is connected to a microwave transmitter23 which sends signal modulation microwaves output by the laser diode 12through a guide 24 connected to the optical guide 13 (which may be ofthe Max-Zehnder type). This emitter 23 produces coupling between thelight from laser diode 12 and the microwaves that it produces bymodifying the impedance of the optical guide 13. Therefore it imposes apulsed emission of light from the laser which makes the equipmentoperate in accordance with the pump-probe method for which thesignal-to-noise ratio is higher.

However, the use of this emitter 23 is only optional, and other elementsdescribed above for this equipment could also be omitted or replaced.

The optical guide 13 could be replaced by an optical fibers network; themicro-lasers 4, assumed so far to be in direct optical contact with thephoto-conducting layers 5 by being placed on these layers or on atransparent substrate of these layers, could be separated from eachother by an optical concentration system.

It is possible and advantageous to place the instrument on an integratedcircuit board P; otherwise, the receiver 2 and the emitter 1 could beplaced on separate boards.

Therefore, the equipment operates when light from the laser diode 12reaches the micro-lasers 4, with the opto-electronic gate 19 having beenopened; the micro-laser 4 of the emitter 1 illuminates a portion of thephoto-conducting layer 5 adjacent to parts 6 and 7 of the antenna 3which creates a short circuit between them and emission of anelectromagnetic wave stream through antenna 3 of emitter 1. This streamis reflected by the medium to be studied or by a mirror located behindthis medium to antenna 3 of receiver 2. It is then collected in the formof an electric signal when the micro-laser 4 of the receiver 2, placedsimilarly to the micro-laser of emitter 1 with respect to thephoto-conducting layer 5 and to parts 6 and 7 of antenna 3, puts theseparts in short circuit; the current created by the wave stream passesthrough the connection lines 22, the amplifier stage 21 and ends up atthe microprocessor 20. The device shown in FIG. 1 can be used to samplethe received signal by means of optical delay lines 16 that illuminatethe micro-laser 4 at successive instants and are therefore used tosample different portions of the received wave stream. Currentsgenerated by these signal portions pass through the correspondingconnection line 22 that stops in front of the optical delay line 16active at that time.

The photo-diode 12 can emit continuously using the energy supplied bythe electricity power supply 11, but this has no effect while theopto-electronic gate 19 is closed; it is decided to control this gate toperiodically open it and thus emit a sequence of wave streams betweenwhich the signals can be received and recorded.

An operating variant consists of making the laser diode 12 emit in apulsed manner by means of appropriate control of the electricity powersupply by the control system 18.

Micro-lasers 4 can be integrated in the emitter 1 and the receiver, bymaking them fixed to the photo-conducting layers 5, so that they can beused easily and without making any adjustments to the position or thefocal length, even if the emitter and the receiver are moved between twotests. Furthermore, micro-lasers are suitable for emitting very shortduration wave streams, for example lasting 1 or 2 picoseconds, whereasdurations of 6 picoseconds are given in the previous U.S. Pat. No.4,855,749 in which a conventional laser is used. The result is thathigher frequency excitations are achieved, and that it is frequentlypossible for emitter 1 and receiver 2 to be coincident if this isdesirable; this would then mean that only a single antenna andmicro-laser support substrate is necessary, to which all electrical andoptical lines in FIG. 1 would lead; reception would then take placeafter emission. All that is necessary is that the path traveled by theemitted wave stream is a sufficiently long path so that the stream doesnot return to the emitter-receiver until after the emission hasfinished.

FIG. 2 illustrates an emitting unit 101 for a slightly differentembodiment of the equipment; the receiving unit may be built in the sameway or with a signal sampler similar to that shown in FIG. 1 (in whichit is also optional).

The antenna 103 is still deposited on a photo-conducting layer 105, anda micro-laser 104 is located behind this layer 105 from which it isseparated by a stack of components; this stack comprises in sequence anoptical trigger 25 in front of the micro-laser 104, a semi-reflectinglayer 26, a micro-lens 27 or another light concentration device, animpedance adaptation layer 28 and finally a transparent and insulatinglayer 29 (that may be made of sapphire or silica) in contact with thephoto-conducting layer 105 which acts as a substrate.

The optical trigger 25 is a bi(4-dimethylaminodithiobenzyl)nickel layerthat provides the luminous power flux for the micro-laser 104 at the endof the layer stack; the semi-reflecting layer 26 prevents light fromreturning to micro-laser 104 and thus closes its oscillating cavitywhile avoiding radiation losses, damage to the control switchgear due toreflected light and even the ends of wave stream produced by doublereflection and which would disturb the measurements; the micro-lens 27concentrating light flux towards the area of interest of thephoto-conducting layer 105 may be a polymer layer such as a melted“photoresist” resin described on pages 1322 to 1324 in the June 1993issue of Optical Engineering; and the purpose of the impedanceadaptation layer 28 is to eliminate reflections of light pulses from themicro-laser 104 produced in front of layer 29.

The micro-lens 27 replaces the ordinary lenses usually used with lasersto concentrate their beam on the target. Since it is integrated into theemitter 101 or the receiver 102, it cannot lose its settings.Concentration of the beam from micro-laser 4 between parts 6 and 7 ofantenna 3 can moderate the total light intensity to be supplied,particularly in that the focal distance of the micro-lens 27 is veryshort. Note also that the intensity of the transmitted wave stream alsodepends very strongly on the material from which the photo-conductinglayer 105 is made, and that silicon doped smith oxygen, which isconventional for this application, may advantageously be replaced bycadmium telluride and gallium arsenide as mentioned above, provided atleast that they are manufactured by making them grow at low temperatureso that they do not produce a single crystal; cadmium telluride is thenformed in polycrystals, and gallium arsenide contains almost purearsenic inclusions. These structural heterogeneities encourage mobilityof electrons in the crystal and therefore conductivity. These advantageswould be expected in other non-monocrystalline crystals, which are stilleasy to make by using a growth temperature that is too low for perfectannealing. Currents 100 times larger (100 pA instead of 1 pA) can beobtained in antenna 3 by using this type of material.

The separate conducting portions 106 and 107 of the antenna 103 arestill strips placed side by side to define a bipole structure, or theymay be spiral, helical, etc., depending on the application consideredand the useful frequency band. In this case, the electromagneticradiation is emitted perpendicular to the strips.

The control system is not modified with respect to FIG. 1, and theemitter 101 and receiver 102 may or may not be on the same supportboard.

FIG. 3 shows that other modifications can be made to the equipment inFIGS. 1 and 2; emitter 201 and receiver 202 are firstly separated toenable the examination of an intermediate gaseous medium without anywave reflection in his medium (it will often be preferred to study solidmedia with a reflection of the wave stream; the device in FIG. 1 or adevice in the combined emitter and receiver will then be used). Thelaser diode 12 can then be replaced by a pair of laser diodes 212, eachof which is located immediately behind the micro-laser 204, and theoptical guide in this case marked as reference 213 connects theelectricity power supply 11 to each of the laser diodes 212. Howeverthis arrangement is not compulsory, and a single laser diode as shown inFIG. 1 could be used, which would be integrated either into the emittersupport board (P1) or into the receiver support board (P2), or into acontrol components support board (P3). This optical guide would then bedesigned appropriately and could only comprise a single optical fiberconnecting the emitter and receiver boards P1 and P2. Free emission oflaser light from one board to the other would also be possible withoutan optical fiber. Electricity power supply, control and recording linesalso depend on the precise layout of elements of the equipment.

The emitter 201, the receiver 202 and their antennas 3 can be aligned,so that the waves would be transmitted directly from one to another; asshown in the figure, it would also be possible to place parabolic orelliptical reflectors 30 in front of the antennas 3 to transform thedivergent waves emitted by antennas 3 into parallel waves or wavesfocused otherwise passing through the medium being studied.

Note also that the photo-conducting layers and the antennas of emitter201 and receiver 202 are laid out facing each other, or that the beamsof micro-lasers 204 that excite them are approximately co-linear (takingaccount of reflections produced by reflectors 30); the advantage is thatthe wave stream emitted from emitter 201 spreads out approximatelyisotropically immediately after leaving emitter 201, passing through thegaseous medium being studied with a uniform front and arrives atreceiver 202 at the same instant; this situation in FIG. 1, in whichemitter 1 and receiver 2 are in the same plane, is different since thewave stream then has an X component that spreads out in the plane commonto emitter 1 and receiver 2, and a Y component that spreads out in anorthogonal plane; these X and Y components propagate in the same Zdirection as in the figure but they are slightly out of phase andtherefore the signal produced is not as sharp as on receiver 202. Thisdisadvantage needs to be considered and balanced with the ease ofmanufacture of the device in FIG. 1 with a single board P, beforechoosing this embodiment or the embodiment shown in FIG. 3.

FIG. 4 shows the shape of an electromagnetic wave stream emitted by theantenna from the emitting unit 1, 101 or 201 as a function of time. TheFourier transform of this wave stream shows that it can be decomposedinto an extended frequency range (on the abscissas axis) as shown inFIG. 6. The signal received by the antenna of receiver 2, 102 or 202 anddisturbed by attenuation or reflection through the medium being studiedmay be like that shown in FIG. 5. Its Fourier transform is given in FIG.7; it is used in the normal manner and in particular may includeabsorption peaks revealing the state of the medium being studied andparticularly the presence of some gases. The example shown representstwo of these peaks at frequencies f1 and f2.

Finally, FIG. 8 illustrates an example of a single emitter-receiver 301according to a previous part of this description; the emitter andreceiver are combined, and the emitter-receiver has an antenna 303composed of two metallic parts 306 and 307 on a photo-conducting layer305, and a micro-laser 304 creates a short circuit between them; lines9, 10, 14, 15 and 22 described with reference to FIG. 1 lead to it inthe same way, and the rest of the equipment is identical to that shownin FIG. 1 except that the shielding 17 is omitted.

What is claimed is:
 1. Wave emitter-receiver device for examining a medium through which waves pass, comprising, in combination, an emitter for emitting emitted waves and a receiver for receiving received waves after passing through said medium, said emitter and receiver, each having an antenna, wherein the emitter antenna and the receiver antenna each are formed of distinct portions laid out on separate photo-conducting layers; a wave generation support device comprising an electricity power supply connected to said portions of said emitter antenna and creating a potential difference between said portions of said emitter antenna; said wave generation support device also comprising a light source including a first micro-laser for supplying first light pulses to the photo-conducting layer of said emitter between said portions of said emitter antenna, said potential difference triggering said emitted waves if said photo-conducting layer of said emitter is illuminated by said light source; a signal reception support device comprising a second micro-laser for supplying second light pulses to the photo-conducting layer of said receiver between said portions of said receiver antenna; and a wave usage device for recording and analyzing said received waves, wherein the photo-conducting layers comprise a non-monocrystalline crystal.
 2. Device according to claim 1, wherein the light source includes micro-lenses for concentrating light pulses between the photo-conducting layers and the micro-lasers.
 3. Device according to claim 2, and further comprising anti-reflection layers inserted between the micro-lasers and the micro-lenses.
 4. Device according to claim 3, and further comprising optical impedance adaptation layers inserted between the micro-lenses and the photo-conducting layers.
 5. Device according to claim 1, wherein the crystal comprises cadmium telluride polycrystal.
 6. Device according to claim 1, wherein the crystal comprises gallium arsenide crystal in which arsenic rich inclusions are formed.
 7. Device according to claim 1, wherein the light pulses supplied have durations of less than 2 ps.
 8. Device according to claim 7, wherein the emitter and the receiver are an integrated device having one antenna formed of distinct portions laid out on one photo-conducting layer and having one micro-laser, said integrated device acting as said emitter during a first time interval and acting as said receiver during a second time interval.
 9. Device according to claim 1, wherein the light source comprises optical delay lines between said receiver and a laser.
 10. Device according to claim 9, wherein the optical delay lines have different delays, and the signal reception device and the wave usage device comprise connection lines capable of sampling signals received by the receiver.
 11. Device according to claim 1, and further comprising a reflector placed in a wave radiation path between the emitter antenna and the receiver antenna.
 12. Device according to claim 1, wherein the emitter and the receiver are located on separate boards including an emitter board including the photo-conducting layer of the emitter, and a receiver board including the photo-conducting layer of the receiver.
 13. Device according to claim 12, wherein the emitter and receiver boards face one another.
 14. Device according to claim 1, wherein the emitter and the receiver are on separate photo-conducting layers and located side by side on a single board with electromagnetic shielding between them, and the emitter and the receiver antennas are oriented in parallel directions to each other.
 15. Device according to claim 1, wherein said electricity power supply is further connected to said emitter.
 16. Device according to claim 1, wherein the light source further comprises a laser and an alternating light cut-off and transmission gate.
 17. Device according to claim 16, wherein the gate comprises a component that changes the optical index as a function of an electric field, said electric field being generated in an adjacent control element.
 18. Device according to claim 1, wherein the light source further comprises a light guide network with a fork towards the emitter and the receiver, said fork comprising a first branch to the emitter and a second branch to the receiver.
 19. Device according to claim 18, and further comprising means for varying the optical impedance of the light guide network. 